U.S. patent number 7,574,075 [Application Number 12/395,788] was granted by the patent office on 2009-08-11 for fiber bragg grating and fabrication method.
This patent grant is currently assigned to General Electric Company. Invention is credited to Hua Xia.
United States Patent |
7,574,075 |
Xia |
August 11, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Fiber Bragg grating and fabrication method
Abstract
A method of fabrication of a thermally stabilized Type I fiber
Bragg grating-based temperature sensing device includes doping a
fiber core material with germanium or germanium oxide for enhancing
photosensitivity, co-doping the fiber core material with fluorine
or chorine or for increasing a mean coordination number; and
ultraviolet laser inscribing a periodic or quasiperiodic modulated
refractive index structure in the fiber core using a laser energy
operating at less than 1000 milliJoules per square centimeter per
pulse. The resulting sensor is operable for more than 1000 hours at
temperatures up to at least 550 degrees Celsius.
Inventors: |
Xia; Hua (Altamont, NY) |
Assignee: |
General Electric Company
(Niskayuna, NY)
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Family
ID: |
40798558 |
Appl.
No.: |
12/395,788 |
Filed: |
March 2, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090169150 A1 |
Jul 2, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11855457 |
Sep 14, 2007 |
7499605 |
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Current U.S.
Class: |
385/12; 385/123;
385/126; 385/37; 438/32; 65/385; 65/390 |
Current CPC
Class: |
G01K
11/3206 (20130101); G02B 6/02114 (20130101); G02B
6/02138 (20130101); C03B 2201/08 (20130101); C03B
2201/12 (20130101); C03B 2201/21 (20130101); C03B
2201/31 (20130101); C03B 2203/18 (20130101); C03B
2203/23 (20130101); G02B 6/02204 (20130101) |
Current International
Class: |
G02B
6/00 (20060101); C03B 37/023 (20060101) |
Field of
Search: |
;385/12,37,31,123,122,124,126,127,128,141,142,143,144 ;438/32
;65/385,390,394 ;250/227.11,227.14,227.18 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Xia Zhang, Jingxi Zhao, Yongqing Huang, Xiaomin Ren; "Analysis of
Shift in Bragg wavelength of Fiber Bragg Gratings with Finite
Cladding Radius"; Proceedings of ICCT2003; pp. 586-589. cited by
other.
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Primary Examiner: Healy; Brian M
Attorney, Agent or Firm: Agosti; Ann M.
Parent Case Text
CROSS REFERENCE
This application is a Continuation in Part of U.S. patent
application Ser. No. 11/855,457, entitled "FIBER BRAGG GRATING FOR
HIGH TEMPERATURE SENSING," filed 14 Sep. 2007, which is herein
incorporated by reference.
Claims
The invention claimed is:
1. A method of fabrication of a thermally stabilized fiber Bragg
grating-based temperature sensing device comprising: doping a fiber
core material with germanium or germanium dioxide for enhancing
photosensitivity; co-doping the fiber core material with fluorine
or chlorine for increasing a mean coordination number; and laser
inscribing a periodic or quasiperiodic modulated refractive index
structure in the fiber core using an ultraviolet laser with a laser
energy operating at less than 1000 millijoules per square
centimeter per pulse.
2. The method of claim 1 wherein laser inscribing comprises using a
laser operating at less than 500 milliJoules per square centimeter
per pulse.
3. The method of claim 1 wherein laser inscribing comprises using a
laser operating at less than 300 milliJoules per square centimeter
per pulse.
4. The method of claim 1 wherein doping comprises doping of
germanium in a range of 7 weight percent to 15 weight percent.
5. The method of claim 4 wherein co-doping comprises doping of
fluorine in a range of 1 weight percent to 5 weight percent.
6. The method of claim 5 wherein co-doping comprises doping of
fluorine at about 2.5 weight percent.
7. The method of claim 4 wherein co-doping comprises doping of
chlorine in a range of 0.5 weight percent to 5 weight percent.
8. The method of claim 1 further comprising thermally annealing the
fiber core material for widening the band gap of the fiber core
material.
9. The method of claim 8 wherein annealing occurs at a temperature
from 300 degrees Celsius to 570 degrees Celsius for a period of
time from 1 hour to 100 hours.
10. A thermally stabilized fiber Bragg grating based sensor
comprising: a fiber core including a plurality of Bragg grating
elements wherein the grating elements comprise a periodic or a
quasiperiodic modulated refractive index structure, the fiber core
is doped with germanium or germanium oxide, the fiber core is
co-doped with fluorine or chorine, and the sensor is operable for
more than 100 hours at temperatures up to at least 550 degrees
Celsius.
11. The sensor of claim 10 further comprising a fiber cladding
surrounding the fiber core in the region of the Bragg grating
elements.
12. The sensor of claim 11 wherein the cladding comprises silicon
dioxide.
13. The sensor of claim 12 wherein the cladding is doped with
germanium, fluorine or chorine, phosphorus, or combinations
thereof.
14. The sensor of claim 10 comprising germanium in a range of 7
weight percent to 15 weight percent.
15. The sensor of claim 10 comprising fluorine in a range of 1
weight percent to 5 weight percent.
16. The sensor of claim 10 comprising fluorine at about 2.5 weight
percent.
17. The sensor of claim 10 comprising chlorine in a range of 0.5
weight percent to 5 weight percent.
18. The sensor of claim 10 wherein the sensor is operable for more
than 100 hours at temperatures in excess of 800 degrees
Celsius.
19. The sensor of claim 10 wherein the fiber core comprises silicon
dioxide.
20. The sensor of claim 10 wherein the sensor comprises a Type I
fiber Bragg grating based sensor.
Description
BACKGROUND
The invention relates generally to fiber Bragg grating based
sensing devices and, more particularly, to thermally stabilized
fiber Bragg grating based sensing devices that can be operated at
elevated temperatures as compared with conventional fiber Bragg
grating based sensing devices.
In general, there are several techniques used for measurement of
parameters such as temperatures. Some of the commonly used systems
include thermocouples and pyrometry and blackbody measurement
devices. A fiber Bragg grating (FBG) based fiberoptic temperature
sensor includes a FBG that is a high quality reflector constructed
in an optical fiber that reflects particular wavelengths of light
and transmits other wavelengths. This is generally achieved by
adding a periodic variation to a refractive index of the fiber. It
is advantageous to use FBG sensors for power generation industrial
process monitoring because of the sensors' low mass, high
sensitivity, and electromagnetic interference immunity, for
example.
However, conventional ultraviolet (UV) light induced FBG sensors
exhibit undesirable thermal instability at elevated temperatures.
UV inscribed FBGs may be of various types with several including
Type I, Type IIA, and Type II. The type typically refers to the
method by which gratings are produced in the fiber. The different
methods of forming the gratings effect physical attributes of the
gratings such as ability to withstand elevated temperatures. The
fibers on which Bragg gratings are formed as well as any associated
claddings of such fibers may be doped or un-doped. In some
embodiments, both the cladding and core are doped. When the fiber
core has no dopant, the fiber cladding is typically doped for
reducing the cladding index of refraction so that the cladding can
confine light wave propagation inside the fiber core. Typical
doping atoms include as phosphorus, boron, fluorine, erbium,
yttrium, aluminum, and tin.
Type I gratings are standard gratings written in both hydrogenated
and non-hydrogenated fibers and are the only types of gratings that
are commercially (off-the-shelf) available. A Type I grating is a
periodic refractive index modulated grating structure and starts to
degrade at temperatures higher than about 300 degrees Celsius after
only a few, for example 2-4, hours of operation. Thus, it is
difficult to use a Type I FBG as a sensor higher elevated
temperature environments.
Type IIA gratings are regenerated gratings that are written after
erasure of a Type I grating. These gratings require an additional
writing step and higher energy levels of the inscribing lasers.
Type IIA gratings generally have higher erasure temperatures than
Type I gratings but require higher laser energy and longer
inscription time. A Type II grating is a damage written grating
inscribed by high power pulsed lasers. Fringes take the form of
physical changes in the crystal lattice. On the other hand, Type II
gratings, inscribed at high power levels, have a broad reflective
spectrum that is generally undesirable for high temperature sensing
applications. The high power pulses required of Type IIA and Type
II gratings are achievable only with expensive processing
equipment.
Therefore, a need exists for a high-temperature operable Type I FBG
based fiber optic sensor and fabrication method that addresses one
or more of the problems set forth above.
BRIEF DESCRIPTION
In accordance with one aspect of the invention, a method of
fabrication of a thermally stabilized fiber Bragg grating-based
temperature sensing device comprises: doping a fiber core material
with germanium or germanium oxide for enhancing photosensitivity;
co-doping the fiber core material with fluorine or chorine for
increasing a mean coordination number; and, with a ultraviolet
laser, inscribing a periodic or quasiperiodic modulated refractive
index structure in the fiber core using a laser energy operating at
less than 1000 milliJoules per square centimeter per pulse.
In accordance with another aspect of the invention, a thermally
stabilized Type I fiber Bragg grating based sensor comprises a
fiber core including a plurality of Bragg grating elements wherein:
the grating elements comprise a periodic or a quasiperiodic
modulated refractive index structure; the fiber core is doped with
germanium or germanium oxide; the fiber core is co-doped with
fluorine or chorine; and the sensor is operable for more than 100
hours at temperatures up to at least 550 degrees Celsius.
DRAWINGS
These and other features, aspects, and advantages of the present
invention will become better understood when the following detailed
description is read with reference to the accompanying drawings in
which like characters represent like parts throughout the drawings,
wherein:
FIG. 1 is a schematic illustration of a band gap of a modified
fiber material in accordance with an embodiment of the
invention.
FIG. 2 illustrates an exemplary fiber optic sensing system using a
fiber optic grating structure in accordance with an embodiment of
the invention.
FIG. 3 is a cross-sectional view of an exemplary configuration of
the fiber optic grating structure in FIG. 2.
FIG. 4 is a diagrammatic illustration of exemplary grating elements
of FIG. 3 including a Gaussian apodized grating profile.
FIG. 5 is a cross-sectional view of an exemplary configuration for
the single thermally stabilized high temperature fiber optic sensor
14 of FIG. 2.
FIG. 6 is a flow chart representing steps in an exemplary
fabrication method.
FIG. 7 is a table of various dopant and co-dopant percentages and
the resulting erasure temperatures for a silica fiber based FBG
sensor.
FIG. 8 is a graph illustrating reflected peak power in dBm versus
temperature in degrees Celsius for three fiber material
samples.
FIG. 9 is a graph illustrating FBG wavelength thermal response in
nanometers versus time in hours for the Ge/F sample of FIG. 8.
FIG. 10 is a graph of FBG peak power in dBm versus time in hours
for the samples of FIG. 8.
FIG. 11 is a graph illustrating wavelength shifts in picometer
versus time in hours for the GE/Cl sample of FIG. 8.
FIG. 12 is a schematic illustration of a gas turbine exhaust duct
as well as FBG sensors in circumferential sensing cables.
FIG. 13 is a graph illustrating power loss in dBm versus wavelength
shift in nanometers for 29 Ge/F co-doped FBG sensors at different
gas turbine startup temperatures.
FIG. 14 is a graph showing measured averaged temperatures of the 29
GE/F co-doped FBG sensors and measured averaged temperatures of 27
inexpensive type K (chromel-alumel) thermocouples.
DETAILED DESCRIPTION
As discussed in detail below, embodiments of the present invention
include a fiber Bragg grating (FBG) based sensing device and a
method of fabricating the same. In one embodiments, a method of
fabrication of a thermally stabilized Type I fiber Bragg
grating-based temperature sensing device includes doping a fiber
core material with germanium or germanium oxide for enhancing
photosensitivity, co-doping the fiber core material with fluorine
or chorine for increasing a mean coordination number, and, with an
ultraviolet laser, inscribing a periodic or quasiperiodic modulated
refractive index structure in the fiber core using a laser energy
operating at less than 1000 milliJoules per square centimeter per
pulse. In one aspect, the fiber sensor is operable for more than
100 hours at temperatures up to at least 550 degrees Celsius. The
various embodiments and materials described herein are useful in
providing a higher temperature sensor than can be obtained by use
of lower power pulses than are used with conventional Type IIA and
Type II gratings and thus can be obtained at less expense.
Referring to the drawings, FIG. 1 is a schematic illustration of a
band gap diagram 500 of a modified fiber material with doped
foreign atoms for functionalizing the fiber to have
photosensitivity and also with co-doping ions to increase fiber
material crosslink mean coordination number while reducing dangling
bonds. The doped atoms and the co-doped ions in the fiber material
form discrete energy levels in between low energy levels or
covalence bands 508 and higher energy levels or conduction bands
510. Carriers 502 for example, electrons and holes, can jump from
the low energy levels 508 to higher energy levels 510.
Dopant and impurities effectively reduce band gap energy of the
fiber material. The loss of the band gap energy may be reduced by a
thermal annealing process as described in aforementioned U.S.
patent application Ser. No. 11/855,457. Such a thermal treatment
process widens fiber core material band-gap from a range between
about 1 eV and about 3 eV represented by reference numeral 506 to a
range between about 4 eV and about 7 eV represented by reference
numeral 507. Some of the impurity and dopant energy levels or
carriers in deep energy levels are eliminated after the thermal
annealing process, so that transfer of the carriers 502 from the
low-level bands and covalence band 508 to the conduction band 510
is less likely even at elevated temperatures. A fiber material
band-gap engineering method may be used to prompt nanostructure and
morphology evolution from a floppy underconstrained amorphous
network to a more compact tetrahedral structure with a higher
coordination number for silicon or germanium atoms. The periodic or
quasi-periodic modulation of the refractive index is kept during
the band-gap engineering method in which the low-refractive-index
area corresponds to low-density tetrahedral clusters, while
high-refractive-index area corresponds to percolative tetrahedral
nanophase structure. Such an alternative nanophase nanostructure
effectively constructs a refractive index modulation in a fiber
core.
FIG. 2 illustrates an exemplary fiber optic sensing system 10 for
detecting multiple parameters in a harsh environment 12. Fiber
optic sensing system 10 includes one or more fiber optic sensors 14
that, in turn, include a fiber Bragg grating structure 16. As
illustrated, sensors 14 are disposed in environment 12, causing
changes in parameters in environment 12 to translate to fiber Bragg
grating structure 16. As discussed in further detail with respect
to FIG. 3, grating structure 16 includes a core that has a
plurality of grating elements having a periodic or a quasiperiodic
modulated Bragg grating structure.
Further, fiber optic sensing system 10 includes a light source 18
that is configured to illuminate grating structure 16. This
illumination facilitates the generation of reflected signals
corresponding to a grating period of grating structure 16. System
10 also includes an optical coupler 20 to manage incoming light
from light source 18 as well as the reflected signals from grating
structure 16. Coupler 20 directs the appropriate reflected signals
to a detector module 22. In an alternative embodiment, the optical
signals may be transmitted through the grating structure, and the
transmitted optical signals are then sensed.
Detector module 22 receives the reflected optical signals from
grating structure 16 and, in cooperation with various hardware and
software components, analyzes the embedded information within the
optical signals. For example, detector module 22 is configured to
estimate a condition or a parameter of environment 12 based upon a
reflection spectrum generated from grating structures 16 of fiber
optic sensor 14. In certain embodiments, detector module 22 employs
an optical coupler or an optical spectral analyzer to analyze
signals from fiber optic sensor 14. Depending on a desired
application, detector module 22 may be configured to measure
various parameters in environment 12. Examples of such parameters
include temperature, the presence of gas, strain, pressure,
vibration, and radiation. The information developed by detector
module 22 may be communicated to an output 24 such as, a display or
a wireless communication device.
FIG. 3 is a cross-sectional view of an exemplary configuration 40
of fiber optic grating structure 16 in FIG. 2. A thermally
stabilized fiber Bragg grating based sensor comprises a fiber core
42 including a plurality of Bragg grating elements 44 that comprise
a periodic or a quasiperiodic modulated refractive index structure.
Fiber core 42 is doped with germanium or germanium oxide and is
co-doped with fluorine or chorine, and the sensor is operable for
more than 100 hours at temperatures up to at least 550 degrees
Celsius. The fiber core may optionally include additional dopants
to the extent such dopants do not detract from its operability
requirements. As used herein, "or" is intended to be inclusive. For
example, "germanium or germanium" oxide means germanium, germanium
oxide, or both. As another example, "fluorine or chorine" means
fluorine, chorine, or both. In a more specific embodiment, the
sensor is operable for more than 100 hours at temperatures in
excess of 700 degrees Celsius. In a still more specific embodiment
the sensor is operable for more than 100 hours at temperatures in
excess of 800 degrees Celsius.
Example fiber materials include silica, silicon dioxide, and
quartz. In one embodiment, fiber core is doped with germanium in a
range of 7 weight percent to 15 weight percent. Fluorine has been
found to be a particularly useful co-dopant and, in one embodiment
is provided in a range of 1 weight percent to 5 weight percent with
a further example being at about 2.5 weight percent. Another useful
co-dopant is chlorine which in one embodiment is used at a range of
0.5 weight percent to 5 weight percent. Another co-dopant which was
found to work at somewhat lower temperatures and higher percentage
ranges is boron in a range of 2 weight percent to 10 weight
percent.
In an optional embodiment, structure 16 includes a depressed
cladding 46 disposed around fiber core 42 in the region of the
Bragg grating elements. As used herein, the term "depressed
cladding" refers to a suppressed cladding mode wherein a fiber
core-cladding interface is fabricated by lightly doping an initial
cladding thickness of about 20 .mu.m to about 40 .mu.m with
fluorine followed by heavily doping rest of the cladding so as to
obtain a difference of about 0.001 in refractive index from that of
the fiber core. In yet another embodiment, the depressed cladding
46 includes silicon dioxide with a fluorine ion dopant. Further, a
primary cladding 48 may be disposed around the depressed cladding
46. In an example, primary cladding 48 includes silicon dioxide.
The use of the depressed cladding 46 is intended to effectively
confine the dopants inside fiber core 42 without diffusing into
primary cladding 48 by elevated temperature. Depressed cladding 46
and primary cladding 48 have a lower index of refraction than that
of fiber core 42 in order to steer light into fiber core 42. Fiber
claddings may comprise silicon dioxide for example and may be doped
or undoped. When doped, several example dopants include germanium,
fluorine or chorine, phosphorus, and combinations thereof.
FIG. 4 is a diagrammatic illustration of exemplary grating elements
44 of FIG. 3 including a Gaussian apodized grating profile. The
apodized grating profile eliminates sub-coherent peaks and sharp
discontinuities for signal processing and peak tracking.
Advantageously, depressed cladding 46 and grating elements 44
confine a guided wave within fiber core 42 to avoid transmission
loss and eliminate high-order coherent interference from adjunct
grating interfaces. Further, cladding wavelength modes that reduce
signal to noise ratio are suppressed. A mean coordination number,
that may be floppy status or highly percolative tetrahedral
clusters or in-between the floppy status and the tetrahedral
cluster, inherently dominates the fiber core material
nanostructure. Regions 45 and 47 correspond to mass density
differences induced by both UV light illumination and thermal
treatment.
FIG. 5 is a cross sectional view of an exemplary configuration 60
for the single thermally stabilized high temperature Type I fiber
optic sensor of FIG. 2. The fiber optic sensor includes a fiber
core 42 fiber Bragg grating structures 44, as referenced in FIG. 2.
An alumina ferrule 62 may be disposed around fiber core 42 and
depressed cladding 46. Alumina ferrule 62 provides mechanical
strength and protection for the high temperature sensor that will
be deployed in the harsh environment. Further, a high temperature
alloy ferrule 64 may be disposed around the alumina ferrule to
provide tolerance to high temperatures. Non-limiting examples of a
high temperature alloys include stainless steel, Inconnel.TM.
austenitic nickel-chromium based superalloys, Invar.TM. nickel
steel alloys, Kovar.TM. nickel-cobalt ferrous alloys, titanium, and
nickel-titanium.
FIG. 6 is a flow chart representing steps in an exemplary method
400 of fiber core material band gap engineering for modifying fiber
material properties. The method 400 includes doping the fiber core
material with one or more atoms for enhancing photosensitivity to
the fiber material in step 402. In a particular embodiment, the one
or more atoms include germanium or germanium oxide. The fiber core
material is also co-doped with one or more ions for enhancing an
amorphous network crosslink of the resulting silicon dioxide and
its bonding mean coordination number in step 404. As used herein,
the term "network crosslink mean coordination number:" refers to
mean silicon atom bonding number or mean coordination number in the
fiber material. In an exemplary embodiment, the one or more ions
include chlorine or fluorine. In another embodiment, the mean
coordination number is enhanced to a range between about 2 to about
2.4. In yet another embodiment, the co-doping of the ions reduces
the dangling bond density.
The method 400 further includes a step 406 of laser inscribing a
periodic or quasiperiodic modulated refractive index structure in
the fiber core using a laser operating at less than 1000
milliJoules per square centimeter per pulse. In a further aspect
the laser inscribing comprises using a laser operating at less than
500 milliJoules per square centimeter per pulse. In still another
embodiment, the laser inscribing comprises using a laser operating
at less than 300 milliJoules per square centimeter per pulse.
Method 400 may optionally further comprise a step 408 of thermally
annealing the fiber core material for widening the band gap of the
fiber core material. In a more specific embodiment, the annealing
occurs at a temperature from 300 degrees Celsius to 570 degrees
Celsius for a period of time from 1 hour to 100 hours.
In one embodiment, the Fiber Bragg grating is hydrogen loaded prior
to inscribing using ultraviolet laser light and phase mask
technology. In another embodiment, the fiber Bragg grating is not
hydrogen loaded prior to inscribing. In yet another embodiment, the
grating is inscribed using pulsed ultraviolet light or near
infrared femtosecond laser to enable a photon-condensation
process.
EXAMPLES
The examples that follow are merely illustrative and should not be
construed to be any sort of limitation on the scope of the claimed
invention.
FIG. 7 is a table of various dopant and co-dopant percentages and
the resulting erasure temperatures for a silica fiber based FBG
sensor. As used herein, the term "erasure" refers to a change in
refractive index of the grating. For a reliable operation in an
elevated temperature environment, it is desirable that the grating
erasure temperature exceed temperatures that are anticipated for
the intended operation. A commercially available photosensitive
fiber core, after being doped and co-doped in the manner described
above with respect to FIG. 6, may be inscribed a UV laser energy of
about 100 milliJoules per square centimeter, for example, to
provide a Type I FBG inscription. As can be seen from the table, by
adjusting the co-dopant atoms, combinations of atoms, and/or weight
percent, the erasure temperature can likewise be adjusted. For
example, the erasure temperature increased as the weight percent
was lowered in the three boron trials, while the erasure
temperature was higher for the increased weight percentage of
fluorine in the two fluorine trials.
FIG. 8 is a graph illustrating reflected peak power in dBm (power
ratio in decibels of the measured power reference to one milliwatt)
versus temperature in degrees Celsius for three fiber material
samples. In this figure the Ge/F sample was Ge 8 wt % and F 2.5 wt
%, the Ge/CL sample was 8 wt % Ge and <0.5 wt % CL, and the Ge/B
sample was 10 wt % Ge and 2 wt % B. Different co-dopants in the
fiber core can improve peak power thermal stability for reliable
operation. In this experiment, the Ge/F co-doped core had a peak
power with good thermal stability in the up to at least 800 degrees
Celsius.
FIG. 9 is a graph illustrating FBG wavelength thermal response in
nanometers versus time in hours for the Ge/F sample of FIG. 8 and
illustrates that the resulting sensor maintains good thermal
stability over time at 550 degrees Celsius. During the experiment,
two cycles were performed to cause the temperature to cycle from
ambient temperature (during which time the waveform valleys
occurred) to 550 degrees Celsius (during which time the waveform
peaks occurred), and an isothermal test at 550 degrees Celsius was
followed for the last 300 hours (during which the waveform peak was
maintained). The resulting data is indicative of repeatability and
survivability.
FIG. 10 is a graph of FBG peak power in dBm versus time in hours
for the three samples of FIG. 8 at 550 degrees Celsius. Once again
this isothermal test indicates that the sensors can be operated at
550 degrees Celsius with negligible reflection power variation.
Thus the FBG sensors of the present disclosure appear to exhibit
thermally stabilized refractive index modulation and thus high
thermal stability.
FIG. 11 is a graph illustrating wavelength shifts in picometer
versus time in hours for the Ge/Cl sample of FIG. 8 and illustrates
that the wavelength shift fluctuation is around +/-15 pm,
corresponding to +/-1.2 degrees Celsius. The small wavelength shift
is mainly dominated by the testing oven thermal variation (K-Type
Thermocouple feedback control), and there is no obvious thermal
decay or drifting indicative of FBG sensor reliability issues. The
nearly drift-free FBG thermal performance indicates that the sensor
is thermally stable.
FIG. 12 is a schematic illustration of a gas turbine exhaust 1202
showing two cables 1206 including FBG sensors 1208 supported by
optional struts 1204. A similar embodiment has been in place in two
gas turbines at a test facility of the assignee for the last 15
months. Fiber sensing arrays with 10 to 50 FBG sensors have been
inscribed on Ge/F co-doped photosensitive fibers and packaged in
stainless steel and Inconnel.TM. tubes to form a fiber sensing
cable. Two half circumferential fiber sensing cables were installed
in gas turbine exhaust ducts in the vicinity of 27 existing type K
(chromel-alumel) thermocouples (not shown). Sensing instrumentation
was connected with a surface cable from the two fiber sensing
cables to fiber optic interrogation units (not shown).
FIG. 13 is a graph illustrating power loss in dBm versus wavelength
shift in nanometers for 29 Ge/F co-doped FBG sensors at different
gas turbine startup temperatures. At the initial temperature of 97
degrees Fahrenheit, 29 peaks can be identified. As the gas turbine
was firing, the exhaust temperature quickly increased. At various
start up temperatures all the FBG sensors or peaks were shifted to
higher wavelengths. Within first 5 minutes of the transient startup
process, the gas turbine exhaust temperature increased from ambient
to 1020 degrees Fahrenheit then back to 660 degrees Fahrenheit as
steady status operation occurred for about 3-6 hours. All the fiber
sensors show transient response to gas turbine startup thermal
dynamics that is similar to existing thermocouple responses.
FIG. 14 is a graph showing measured averaged temperatures of the 29
GE/F co-doped FBG sensors and measured averaged temperatures of 27
inexpensive type K (chromel-alumel) thermocouples. FIG. 14
illustrates that over a temperature range of 1000 degrees
Fahrenheit the measured temperatures of the GE/F sample of FIG. 8
are consistent with the measured temperatures of inexpensive type K
thermocouples that were also positioned in a gas turbine of the
type shown in FIG. 12. By comparing transient and steady status gas
turbine operation, it is shown that both fiber sensors and
thermocouples are consistent to each other in the measured exhaust
temperature. When 29 FBG sensors are packaged in half
circumferential sensing cables, increased spatial sensing
resolution can be obtained in comparison to conventional
thermocouples embodiments without requiring more penetration for
installation and additional bulky electric wires and
high-temperature sheath materials.
The various embodiments of a fiber optic system and fabrication
method described above thus provide a way to achieve, efficient and
accurate measurement of parameters in higher temperature
environments than conventional Type I FBG sensors without the cost
of high power laser inscribing equipment.
While only certain features of the invention have been illustrated
and described herein, modifications and changes will occur to those
skilled in the art. It is, therefore, to be understood that the
appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the invention.
* * * * *